The present invention relates to an apparatus for producing deionized water, comprising an electrodialyzer having cation exchange membranes and anion exchange membranes alternately arranged to form demineralizing compartments and concentrating compartments and having ion exchangers accommodated in the demineralizing compartments, and designed to apply a voltage while supplying water to be treated to the demineralizing compartments.
As an apparatus for producing deionized water, it is common to obtain deionized water by passing water to be treated through a bed packed with an ion exchange resin so that impurity ions are removed as adsorbed on the ion exchange resin. Here, it is common to employ a method of regenerating the ion exchange resin having its adsorbing ability lowered, by means of an acid or alkali. Accordingly, this apparatus has a problem that a waste liquid of the acid or alkali used for the regeneration, is discharged. Therefore, an apparatus for producing deionized water which requires no such regeneration, is desired. From such a viewpoint, an attention has been drawn in recent years to an apparatus for producing deionized water by self-regenerating type electrodialysis wherein ion exchange resins are used in combination with ion exchange membranes. This apparatus comprises an electrodialyzer having anion exchange membranes and cation exchange membranes alternately arranged to form demineralizing compartments and concentrating compartments and having ion exchangers accommodated in the demineralizing compartments, and is designed to apply a voltage while supplying water to be treated to the demineralizing compartments to carry out electrodialysis to produce deionized water. With respect to this conventional apparatus, various developments or improvements have already been made. For example, there have been proposals for an apparatus wherein the width and thickness of the demineralizing compartments have been specified (JP-A-61-107906), an apparatus designed to permit water to be treated to pass at least twice through demineralizing compartments of a deionizing apparatus (JP-A-1-307410), and an apparatus wherein anion exchange resins are used as ion exchange resins to be packed in a portion where the water to be treated passes firstly (JPA-4-71624).
Ion exchange membranes to be used in such conventional apparatus for producing deionized water, were in a wet state and accordingly difficult to handle at the time of setting (or incorporating) in an electrodialyzer, and they had a drawback that they tend to shrink as they are gradually dried during the setting. To solve such a problem, it is conceivable to set them in a dry state, but the problem can not be solved simply by attempting to set the membranes in a dry state without any elaboration. Namely, the ion exchange membranes will be in contact with wet state ion exchange resins present adjacent thereto, whereby the ion exchange membranes will be wetted, and the above mentioned problem of difficulty in handling wet membranes can not be solved. Further, the membranes will deform due to swelling when water is supplied to the electrodialyzer, whereby defects such as creases are likely to form on the membranes. Also from this viewpoint, the merits in setting the membranes in a dry state have been negated.
It is an object of the present invention to solve such a problem and to provide an apparatus for producing deionized water, whereby deformation of ion exchange membranes can be suppressed even if they are set in a dry state, and a constant performance can be obtained.
The present invention provides an apparatus for producing deionized water, comprising an electrodialyzer having cation exchange membranes and anion exchange membranes alternately arranged to form demineralizing compartments and concentrating compartments and having ion exchangers accommodated in the demineralizing compartments, and designed to conduct an electric current while supplying water to be treated to the demineralizing compartments, wherein a spacer is arranged in each concentrating compartment to maintain the thickness of the concentrating compartment, and the cation exchange membranes or the anion exchange membranes are incorporated in their dry state in the electrodialyzer, whereby water is supplied to the electrodialyzer, while each ion exchange membrane is brought in contact with the spaced by a pressure from the demineralizing compartment side.